The insulin trap: A and B chains are NOT made in one cell.
When NEET asks about recombinant insulin (Humulin), the most common mistake is assuming both polypeptide chains are produced together inside a single E. coli cell. They are not.
What NCERT says (Class 12 Biology, Chapter 11, page 242):
Human insulin consists of two polypeptide chains — chain A (21 amino acids) and chain B (30 amino acids). These are linked by disulfide bridges. In the human pancreas, insulin is initially synthesised as proinsulin — a single polypeptide that includes chain A, chain B, and a connecting C-peptide. The C-peptide is removed during maturation.
The biotechnology approach:
Eli Lilly produced Humulin in 1983 using a key strategy: the genes for chain A and chain B were introduced into two separate E. coli strains. Each strain expressed only one chain. The chains were then extracted, purified, and combined in vitro, where disulfide bonds formed between them to produce functional insulin.
Why not express proinsulin directly? Early E. coli systems could not reliably process the C-peptide removal. Separate expression avoided this folding problem entirely.
What NEET targets:
Questions test whether you know: (a) insulin has two chains linked by disulfide bonds, (b) recombinant production uses separate bacterial cultures for each chain, (c) assembly happens outside the cell, and (d) the distinction between proinsulin (with C-peptide) and mature insulin (without it).
Watch out: A distractor claiming "A and B chains are co-expressed and self-assemble inside E. coli" exploits the gap between knowing the chain structure and knowing the production method. The assembly is extracellular — that is the fact NEET rewards.
Select an option to see the explanation. Wrong answers show why your choice was tempting — and name the exact trap it exploits.
Human insulin consists of how many polypeptide chains, and what connects them?
What is the C-peptide in the context of insulin biosynthesis?
Which company first produced recombinant human insulin (Humulin) commercially?
In Eli Lilly's production of Humulin, the A and B chains of insulin were:
Why were the A and B chains of recombinant insulin expressed in separate *E. coli* cultures rather than as a single proinsulin polypeptide?
A student claims: "Proinsulin and mature insulin are identical molecules." Which correction is accurate?
If a biotechnology company wanted to produce functional human insulin by expressing proinsulin as a single polypeptide in *E. coli*, what additional processing step would be essential before the product could be used therapeutically?
Consider the following statements about recombinant insulin production:
Given
A question states: "In the production of recombinant human insulin, Eli Lilly expressed the A and B polypeptide chains of insulin in two separate *E. coli* strains. Explain why this approach was used instead of expressing proinsulin directly, and describe how functional insulin is obtained from the separately produced chains."
Required
Explain the rationale for separate-chain expression and describe the *in vitro* assembly process.
Concept
Human insulin is initially produced in the pancreas as proinsulin — a single polypeptide containing chain A (21 amino acids), chain B (30 amino acids), and a connecting C-peptide. Mature insulin is formed when the C-peptide is enzymatically removed and disulfide bonds link the two chains. Early *E. coli* expression systems lacked the post-translational machinery to cleave the C-peptide correctly.
Formula/principle
No mathematical formula applies. The governing principle is: *E. coli* can transcribe and translate eukaryotic cDNA but cannot perform complex post-translational processing (C-peptide excision).
Substitution/application
Eli Lilly introduced the chain A gene into one *E. coli* strain and the chain B gene into a separate strain. Each culture produces its respective polypeptide independently.
Calculation/reasoning
After expression and extraction: 1. Chain A and chain B are purified separately. 2. The purified chains are mixed under controlled oxidising conditions *in vitro*. 3. Disulfide bonds form between the correct cysteine residues, producing the two-chain insulin molecule. 4. The product (Humulin) is structurally identical to native human insulin.
Final answer
The two-strain approach was necessary because early *E. coli* could not process proinsulin's C-peptide. By expressing chains separately and assembling them extracellularly via disulfide bond formation, Eli Lilly produced functional human insulin identical to the native hormone.
Common trap
The common mistake is claiming that both chains are expressed together in one *E. coli* cell and self-assemble intracellularly. The assembly is extracellular (*in vitro*) — this is the fact NEET distractors exploit.
Similar NEET-style question
"Assertion (A): Recombinant insulin produced by *E. coli* is structurally identical to human insulin. Reason (R): Both A and B chains are co-expressed in the same *E. coli* cell, allowing natural folding. Choose the correct option." Answer: A is true, R is false. The chains are expressed in separate *E. coli* strains and combined *in vitro*. The product is indeed identical to human insulin, but the reason stated is incorrect. ---
Recombinant insulin: Eli Lilly 1983 (Humulin) — A + B chains in E. coli, joined by disulfide. Gene therapy: ADA-deficient SCID (1990, Ashanti DeSilva). ELISA: enzyme-linked immunosorbent assay (HIV diagnosis). Molecular diagnosis: PCR for early detection.
-- NCERT Class 12 Biology, Ch. 11, p. 242These are the exact patterns that cause wrong answers in NEET. Each trap includes when it triggers and how to avoid it.
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Eli Lilly's Humulin: A and B chains expressed SEPARATELY in two E. coli strains; chains then combined in vitro and joined by disulfide bonds.
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26 questions from NEET 2020, 2021, 2022, 2023, 2024, 2025. Answers verified against NTA official keys.
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¬˝ÁÃ’ œŸ ∞ ¡Êß◊Ê ∑ § Áfl ÿ ◊ ª‹Ã ∑§ÕŸ ∑§Ê ¬„øÊÁŸ∞– (4) After zygote formation
Recurring question shapes from past papers. Each pattern shows why wrong options look tempting.
similar term confusion
Biology relies on precise terminology; close terms tempt selection.
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